Revert of Remove enable_vulkan from build/config, and move it to the buildflag_header system.

gpu/vulkan/features.gni

Index: gpu/vulkan/features.gni
diff --git a/gpu/vulkan/features.gni b/gpu/vulkan/features.gni
deleted file mode 100644
index f02ac88abfff9ecb60b1e08a8839c6c5b12a6ad8..0000000000000000000000000000000000000000
--- a/gpu/vulkan/features.gni
+++ /dev/null
@@ -1,10 +0,0 @@
-# Copyright 2017 The Chromium Authors. All rights reserved.
-# Use of this source code is governed by a BSD-style license that can be
-# found in the LICENSE file.
-
-# Features used by targets inside and outside of |gpu/vulkan|.
-# For details see declare_args() in build/config/BUILDCONFIG.gn.
-declare_args() {
-  # Enable experimental vulkan backend.
-  enable_vulkan = false
-}